the Sun
The Sun is the one star we live next to, the bright ball of glowing gas that lights and warms the Earth. Every other star in the night sky is a sun too, but they are so far away that they shrink to points of light. Because the Sun is close, we can study it in detail and use what we learn as a blueprint for understanding the billions of stars we will never visit.
By human standards the Sun is enormous: about 1.39 million km across, roughly 109 Earths laid side by side, and about 333,000 times the mass of the Earth. It sits about 1.5 x 10^8 km away (one astronomical unit), so its light takes a little over 8 minutes to reach us. Its visible surface glows at about 5,500 degrees Celsius, and it pours out energy at a steady rate of about 3.8 x 10^26 watts. By the standards of stars, though, it is ordinary: a middle-sized, middle-aged yellow star roughly 4.6 billion years old, about halfway through its hydrogen-burning life.
The Sun matters because it is the only star we can examine up close, in real time, across its whole surface and even into its interior. Sunspots, flares, the wind it blows, and the way it quivers all give us direct tests of the physics of stars. When astrophysicists say a distant star is 'a Sun-like star' or measure its mass in 'solar masses', they are anchoring the whole science of stars to this one familiar object.
When you measure a distant star's mass as '0.8 solar masses', you are saying it weighs eight-tenths of our Sun — the Sun is the standard ruler.
Solar masses, solar radii, and solar luminosities are the everyday units of stellar astrophysics.
The Sun is not on fire in the chemical sense — nothing is burning the way wood or gas burns. Its energy comes from nuclear fusion deep in its core, a completely different process.